1. Field of the Invention
The present invention relates generally to electronic conductor elements and more particularly to electronic brushes for use in making electrical contact between stationary and moving surfaces.
2. Description of the Prior Art
Electronic brushes are used in a wide variety of electronic devices including computers, incoders, automatic control systems, alarm systems, trimmers, precision potentiometers and the like. In all of these devices, many of which embody several individual brush components, the brush performs the important function of making reliable electrical contact between the various stationary and moving surfaces of the devices.
With the great strides made in recent years in the development of solid state devices and printed circuitry and the concomitant miniaturization of electronic devices, the design and fabrication of electronic brushes, and particularly microminiature brushes, has taken on increased significance. This is true because in many types of electronic devices it is the brush more than any other single component which governs the over-all size and functional precision of the device. Where the surface over which the brush must make electrical contact is miniaturized, the brush itself must, of course, be correspondingly miniaturized. At the same time, however, the brush must meet rigid dimensional tolerances, must be capable of making effective electrical contact with the often irregular surfaces of the circuitry with which it cooperates, and, very importantly, must be constructed so as not to damage the surfaces with which it repeatedly comes in contact during the operation of the electronic apparatus. The ideal brush capable of satisfying these diverse requirements is one that has a great multiplicity of extremely fine contact segments, or fingers, each of which is very flexible yet rugged and each of which has a smooth or coined end contact portion. Experience has shown that the greater the number of small diameter, hair-like flexible fingers in any given brush width, the greater will be the likelihood of the brush making reliable electrical contact with the mating surface and the less will the chances be of undesirable arc erosion and circuit board wear.
In the past, electronic brushes were typically fabricated from a wide variety of metal alloys using generally standard tool and die techniques. The individual brush segments were generally formed by making several biforcations or slits in sheet metal material which had been cut to the desired dimensions. Because of size and mechanical limitations in the die forming apparatus, however, the number of segments or fingers which could be formed on miniaturized brushes was severely restricted.
In an attempt to overcome the limitations inherent in standard die fabrication techniques, the so-called "wire wound" method of brush construction was recently developed. This method basically consists of closely winding an appropriate metal alloy wire having a diameter of three- or four-thousandths of an inch onto a fiberglass drum approximately six inches in diameter and then electroforming a plurality of silver bars at right angles to the wires at spaced intervals around the drum. The matrix thus formed is then cut longitudinally of the drum, removed, and flattened into a planar sheet consisting of a plurality of wires interconnected at spaced intervals by the silver bars. The planar sheet is next cut into elongated strips each having a width equal to from 10 to 25 wires. Individual brushes are then formed by cutting the wires intermediate of the connecting silver bars. Where desired, the end portions of the brushes can then be formed in a forming die into the desired cross-sectional configuration.
Although the wire wound method has been demonstrated to be superior to prior art techniques for the fabrication of small multifid electronic brushes, several serious deficiencies have been found to exist in the brushes produced by this method. For example, because of silver creepage during the plating process, silver is deposited between the strands of wire. This causes excessive spreading of the fingers when the wires are cut to form the electronic brush and may result in a failure of the brush to meet critical dimensional tolerances on brush width. If close width tolerances are not met, the brush will improperly register with the electrical circuitry with which it cooperates and performance of the device will be degraded. Additionally, the silver plating on the individual fingers causes them to be less flexible and further contributes to poor brush performance. Also because of the stress formed in the wire during the coiling operation, after the wires are cut to form the brush the individual fingers tend to curl or otherwise deform beyond acceptable dimensional tolerances.
The unique design of the apparatus of the present invention permits the automatic production of precision microminiature electronic brushes comparable in size to the brushes formed by the wire wound method, at a fraction of the cost of the wire wound brushes. With the apparatus of the present invention, brushes are produced by a fully automatic process directly from a continuous strip of the metal alloy material. The time consuming and costly electroforming step of the wire wound method is completely eliminated, as is the silver creepage problem inherent in the wire wound process.
Additionally, because in the method of the present invention the brush segments or fingers are formed while the material is securely encapsulated within the apparatus in such a manner as to prevent any lateral deformation, undesirable built-in stresses in the material are eliminated and extremely close dimensional tolerances can consistently be maintained. The time consuming and costly hand operations of stripping the silver bar wire matrix from the plating drum and cutting it to size are also eliminated thereby contributing significantly to increased production rates and over-all cost reductions.